Stainless steels, Ti and Ti alloys which are soft substrates capable of being worked easily have been widely used for watchcases, watchbands, necklaces, earrings, pierced earrings, rings, eyeglass frames, pendants, brooches, bracelets and other decorative parts. However, it is indicated that decorative parts obtainable by working these soft substrates have an important problem of deterioration in appearance quality caused by occurrence of flaws during the use thereof. This deterioration is mainly caused by a low surface hardness in soft substrates, namely a low Vickers hardness Hv of about 200. In order to solve the deterioration in appearance quality, various kinds of surface hardening treatments have been attempted.
Furthermore, the decorative parts need to have high decorative properties, and sophisticated pink color is preferred as decorative parts. Surface hardening treatment techniques for securing pink color have been attempted.
As a pink decorative part, an exterior part obtainable by forming a pink alloy coating film containing palladium (Pd) in a weight ratio of 1 to 25% on a pink titanium carbonitride coating film is disclosed (Patent document 1). This prior art discloses that about 1 μm of a pink carbonitride is formed by an ion plating method and thereafter about 0.1 μm of an Au alloy containing 10% of Pd is formed. Furthermore, about 1 μm of a pink Ti carbonitride is formed by an ion plating method and thereafter 0.05 μm of a copper coating film is formed and then 0.1 μm of an Au—Pd alloy is formed by a wet plating method to prepare the decorative part. That is to say, since Ti carbonitride is hard, excellent in flaw resistance and pink, but has low brightness and is dark, a pink Au alloy coating film having high brightness is formed thereon and thereby the flaw resistance is maintained.
Moreover, a method such that 0.5 μm of a Ti nitride film is formed on the surface of a substrate by ion plating, and 0.3 μm of co-deposited film of Ti nitride and Ag or Cu is formed by ion plating and further 0.2 μm of an Au—Pt pink gold film is formed by wet plating is disclosed (Patent document 2). In this method, the pink Au alloy film is formed on the hard co-deposited film of Ti nitride and Ag or Cu and thereby the flaw resistance is maintained.
Patent document 1: JP-A-561 (1986)-127863 (p. 3)
Patent document 2: JP-A-563 (1988)-53267 (p. 4)